In the field of optical materials, one of the critical needs has always been the provision of optically transparent glasses in which the refractive index reaches large values. High refractive index allows glass to bend transmitted light by large angles. The ability to achieve high refractive index is important in design of optical components such as lenses and prisms, and is especially valuable for optical matching, with minimal loss, to crystalline materials. Among the established oxide glasses known to obtain satisfactory optical quality, the refractive indices as listed in the Handbook of Physics and Chemistry usually lie in the range 1.6-1.7: only the heaviest flint glass has n.sub.D =1.87.
Another important property of an optical glass is that it should have a high softening temperature, so that it can remain stable in its dimensions despite excursions to quite high temperatures. It should not have any physical ageing processes which would allow the structure to slowly change thereby allowing critical design dimensions to change. It should be resistant to air and atmospheric moisture so that optical surfaces are not degraded during normal utilization. Still another important property that such an optical glass should have is a high hardness, so that an optical surface, once formed, will successfully resist damage on impact with hard objects and resist abrasion by airborne dust particles which typically have hardnesses in the range of ordinary silicate glasses. The material which combines all these properties in an extreme form is, of course, crystalline diamond. No glassy materials have heretofore been known in which all these desirable properties occur together.
Very large values of the refractive index, even greater than n=2.0, have been obtained by the use of elements like selenium or tellurium, as the major species in the glass. However, these glasses are semiconducting in nature and are black in color, i.e., they absorb light in the visible region of the spectrum, and are suitable only for infrared optics applications. These high index glasses also have very low softening temperatures and hardnesses, which are serious disadvantages in most applications.
Better properties are obtained by replacing the selenium and tellurium with sulfur. Such sulfide glasses may also retain the optical transparency of conventional oxide glasses, hence are attractive for special purpose applications. However, these ionic sulfide glasses are much more prone to attack by atmospheric moisture than are the oxide glasses and suffer from much lower softening temperatures and lower hardnesses than do the oxide glasses.
Attempts to solve the problem posed by lack of optically transparent glasses of high refractive index have also been made by altering the composition of oxide glasses so as to include large proportions of heavy metal cations and heavy metal glassformers. For instance, the calcium oxide of normal "soda-lime silicate" glass is replaced by lead (and, indeed, as a major component in the well known lead crystal glasses is responsible for their aesthetic qualities which are a direct consequence of the high refractive indices of those glasses). In other variations, the sodium oxide is replaced by silver or thallium oxide, while the "glassformer" SiO.sub.2 is replaced by oxides such as Bi.sub.2 O.sub.3 or As.sub.2 O.sub.3. By such replacements, oxide glasses with extremely high refractive indices have been obtained (see, e.g. W. Dumbaugh and J. Laupp, in the J. American Ceramic Soc., vol. 75, pp. 2315-2326 (1992)). However, the high refractive index is obtained in these oxide glasses at the cost of greatly lowered softening temperatures, generally in the range 300.degree.-450.degree. C. One glass of this type containing germanium oxide along with lead and bismuth oxide was shown to have a softening temperature of 470.degree. C. but the refractive index was decreased to 1.94.
Further attempts to improve the glass properties have been made by replacing the oxide ions in the silicate or phosphate glass-forming system by nitride ions, as described in U.S. Pat. No. 4,070,198, Chyung et al. and by S. Sakka, in the Annual Review of Materials science, vol. 16, p. 29 (1986). This procedure has the desired results of simultaneously raising the glass transformation temperature and hardness of the glassy material, along with its refractive index. However, none of these previous efforts was able to replace more than 20-25 atomic per cent of the initial oxygen anions with nitrogen.
Thus it is readily apparent that a great need exists for the development of a new type of glassy material in which all four of the desiderata listed above: high refractive index, high softening temperature, resistance to air and moisture, and high hardness, are obtained. It is towards this goal that the present invention is directed. A preferred embodiment of the present invention obtains all of the desiderata listed above, and furthermore may in principle be produced by methodology much simpler than those previously used to produce currently available examples of the material.